The New York Times has a fascinating story today about a study focusing on the composition of breast milk. Researchers have found that breast milk contains complex sugars that protect babies from harmful bacteria. Put on your science hats for a minute now… here’s a link to the actual study in the Proceedings of the National Academy of Sciences. And boiled down into plain english, is the story from the New York Times:

A large part of human milk cannot be digested by babies and seems to have a purpose quite different from infant nutrition — that of influencing the composition of the bacteria in the infant’s gut…

The details of this three-way relationship between mother, child and gut microbes are being worked out by three researchers at the University of California, Davis — Bruce German, Carlito Lebrilla and David Mills. They and colleagues have found that a particular strain of bacterium, a subspecies of Bifidobacterium longum, possesses a special suite of genes that enable it to thrive on the indigestible component of milk.

This subspecies is commonly found in the feces of breast-fed infants. It coats the lining of the infant’s intestine, protecting it from noxious bacteria…

The indigestible substance that favors the bifido bacterium is a slew of complex sugars derived from lactose, the principal component of milk. The complex sugars consist of a lactose molecule on to which chains of other sugar units have been added. The human genome does not contain the necessary genes to break down the complex sugars, but the bifido subspecies does, the researchers say in a review of their progress in today’s Proceedings of the National Academy of Sciences.

The complex sugars were long thought to have no biological significance, even though they constitute up to 21 percent of milk. Besides promoting growth of the bifido strain, they also serve as decoys for noxious bacteria that might attack the infant’s intestines. The sugars are very similar to those found on the surface of human cells, and are constructed in the breast by the same enzymes. Many toxic bacteria and viruses bind to human cells by docking with the surface sugars. But they will bind to the complex sugars in milk instead. “We think mothers have evolved to let this stuff flush through the infant,” Dr. Mills said…

“We were astonished that milk had so much material that the infant couldn’t digest,” Dr. German said. “Finding that it selectively stimulates the growth of specific bacteria, which are in turn protective of the infant, let us see the genius of the strategy — mothers are recruiting another life-form to baby-sit their baby.”…

The story goes on to say that the researchers are investigating how their findings could be used to help premature infants and the elderly.